The present invention relates to diaphragm metering pumps. Specifically, an apparatus for monitoring and controlling the extension of a diaphragm being actuated via a hydraulic fluid in a metering pump is described.
Metering pumps find diverse uses in many industrial processes. Diaphragm metering pumps operate from flexure of a flexible diaphragm which applies pressure to a pumped media, forcing the media through an outlet check valve. Reduction of the hydraulic pressure against the diaphragm returning to its preflexed state results in the diaphragm creating a pressure differential between the pumping chamber and pumping media inlet. A second valve permits additional pumping media to fill the pumping chamber.
The different applications for these metering pumps require diaphragms as diverse as stainless steel and Teflon. A major source of failure for metering pumps of this type results when the diaphragm ruptures, through excessive flexure and overextension. The overextension of a diaphragm results when the hydraulic force applied to the diaphragm either pushes or pulls it beyond material specific flexural limits.
Limitations against overextension of the diaphragms in either direction are provided by first and second dish plates in the hydraulic fluid chamber and pumping chamber. An overextension condition will occur as a result of a hydraulic imbalance as can be caused by leakage of hydraulic fluid past the piston. During retraction of the piston, which produces the hydraulic force for actuating the diaphragm, the diaphragm retracts against the rear dish plate before achieving an overextended state. Likewise, when the diaphragm is in the forward extended position during forward extension of the piston, a forwardly located dish plate retains the diaphragm from achieving an overextended state. Contact of the diaphragm with the dish plate can result in excessive stress levels and can contribute to pre-mature diaphragm failure and is therefore, undesirable.
The subject of monitoring diaphragm failure has been described in several prior art patents. In U.S. Pat. No. 4,781,535 to Mearns, a leak detector was provided which essentially detected the occurrence of a rupture in the diaphragm after the fact. Although this technique minimizes the amount of contamination which results from hydraulic fluid mixing with pumped media and otherwise signals corrective action at the earliest possible time, it does not control diaphragm deflection to be certain that the deflection is within safe limits to avoid the possibility of a rupture and to prolong the life of a diaphragm.
The sensing of diaphragm position has been considered in U.S. Pat. Nos. 4,619,589 and 4,828,464. In these devices, the position of the diaphragm is monitored in an effort to precisely control the amount of fluid being pumped. The problem of overextension of the diaphragm in both directions, however, has not been completely addressed by the prior art. Experience has shown that the rearward dish plate will cause extrusion of some diaphragm materials such as Teflon when the diaphragm is drawn against the porous dish plate when the piston is retracted. Further, cavitation has been experienced wherein an air interface occurs between the diaphragm and hydraulic fluid in some extreme circumstances, due to the dish plate inhibiting further rearward movement of the diaphragm. The cavitation effect reduces the metering accuracy of the pump and is otherwise undesirable.
Given the foregoing difficulties of maintaining metering pump reliability, the present invention has been provided.